The field of the disclosure relates generally to alternating current (AC) motors, and more specifically to an AC motor with a drive circuit.
At least some known AC motors are fixed speed motors that are driven by applying line input voltage directly to the motor through a contactor or relay in an across-the-line architecture. Such AC motors are most efficient when operating at full speed and at line frequency. During low load conditions where full speed operation is not necessary, use of the line input voltage causes such AC motors to be operated at a higher power level than necessary, which reduces efficiency. Variable frequency drives (VFDs) enable driving AC motors at variable speeds within the motors' operating ranges. This variable speed control using VFDs increases the efficiency of operating the AC motors at less than full speeds, as compared to the across-the-line architecture. However, such VFDs are expensive and may even exceed the cost of the AC motor in some cases. Additionally, using a VFD to controllably increase voltage magnitude up to the AC motor's rated voltage or horsepower may require a large-sized VFD having a voltage or horsepower rating that is substantially equal to that of the AC motor. VFD's add approximately 3-5% more losses that a motor-only design would not have when running off of line voltage. Because of this increase of loss, to maintain the same high-speed efficiency as a system having a VFD, the active material content in the motor design must increase. This may include increasing a frame size of the motor or increasing a length of the existing frame by adding additional windings and steel material.
Single-speed AC induction motors are commonly used in air and water moving applications due to their low cost and high efficiency when operated at line frequency. Recent environmental regulations and standards require lower speed operation of motors used in the air and water moving applications to improve efficiency. One method of providing lower speed operation includes adding a second winding to a single-speed AC induction motor, to provide high speed operation when using the first winding and low speed operation when using the second winding. However, adding a second winding typically requires increasing the frame size of the AC induction motor, which is cost-prohibitive. Another method includes adding a VFD to the AC induction motor, which as described above, adds significant size and cost to the motor.